Guns...
We do have a historical model from the very same time period of the effectiveness of Japanese heavy artillery against an Allied fortress.. in this case, Corrigedor, which had several batteries of heavy 12 inch mortars that it used as counter battery artillery against Japanese heavy seige guns located on Bataan.
During the months after the fall of Bataan leading up to the landing, the Japanese using artillery, with some (it tended to not be very effective) bombing silenced every one of these US pieces, which were in open topped mounts (although well protected from naval gunfire, not so much from howitzer fire and bombers). Although US guns did occasionally do some serious damage to the Japanese, the dense jungle terrain and lack of aerial spotting (because the Japanese dominated the air) made accurate counter battery fire impossible.
I see the same situation likely to develop at Singapore, particularly as Singapore is much more vital to the Japanese then Manila Bay. They wanted the Philippines to keep the US from interfering with Japanese moves against Malaya and Dutch East Indies, while Singapore was vital in order to launch the move against Sumatra and Java.
So no other points of departure, the Japanese bring up heavy guns and silence the British guns after an impressive artillery duel as the Japanese will have air superiority and aerial observation, while the British will not.
The post above alluding to the domino effect however is dead on... it would take time to then shift these guns to the Philippines, which means that Corregidor has to be starved out (it will fall though, it had a mere week or so food left when the Japanese landed). It would delay the Dutch East Indies campaign by several important weeks I would think, which delays the invasion of Burma, threat to India (including the Bay of Bengal Raid), also delays the assault on the Solomons and might even butterfly out the Battle of Coral Sea (with Yamamoto throwing all 6 carriers at Midway instead, which would see 4 US carriers vs 6 Japanese, but no other significant force changes).